President Obama arrives Sunday in Paris to finalize a global climate-change pact that if completed would be a legacy-defining part of his presidency. But he awaits challenges at home and abroad, including questions about who will pay for the changes and whether terrorism is a more imminent concern.

On Capitol Hill, Senate Republicans suggested last week that the GOP-led chamber must approve the Paris deal, or it will withhold billions that the U.S. has pledged, as part of the pact, to help poor countries reduces their carbon output.

“Congress will not be forthcoming with these funds in the future without a vote in the Senate on any final agreement as required in the U.S. Constitution,” Oklahoma Sen. James Inhofe, chairman of the Senate Environment and Public Works Committee, and 36 other GOP senators said in a letter to Obama.

They also made clear that any deal including taxpayer money and a binding timetable on emissions must have Senate approval. And they argue that Obama has already pledged $3 billion to the Green Climate Fund “without the consent of Congress.”

The United Nations talks will take place on the outskirts of Paris, where 130 people were killed roughly two weeks ago in terror attacks, which has also sparked concerns about whether world leaders should now be more focused on stopping terror groups.

Obama said Tuesday at a White House press conference with French President Francois Hollande that the summit will be a “powerful rebuke” to terrorists, including the Islamic State, which has claimed responsibility for the Paris attacks.

“The world stands as one and shows that we will not be deterred from building a better future for our children,” Obama also said.

Still, Paris and the surrounding area will essentially be locked down for the 12-day summit. And climate-change activists have reportedly agreed to cancel a march Sunday, after an appeal from French leaders.

“I have to salute the responsibility of the organizations who would have liked to demonstrate but who understand that if they demonstrate in a public place there is a security risk, or even a risk of panic,” French Foreign Minister Laurent Fabius told The Guardian.

About 150 heads of state are set to join Obama for talks on Monday and Tuesday as the deal nears the finish line. The goal is to secure worldwide cuts to emissions of heat-trapping gases to limit the rise of global temperatures to about another 2 degrees from now.

The concept behind a Paris pact is that the 170 or so nations already have filed their plans. They would then promise to fulfill their commitments in a separate arrangement to avoid the need for ratification by the U.S. Senate.

Such dual-level agreement could be considered part of a 1992 treaty already approved by the Senate, said Nigel Purvis, an environmental negotiator in the Clinton and George W. Bush administrations.

But it's not just about whether or not to ratify.

Latin America countries attending the negotiations reportedly will demand that the wealthiest countries and those that pollute the most pay for the reduction of carbon emissions.

In the United States, the talks are entangled in the debate about whether humans really are contributing to climate change, and what, if anything, policymakers should do about it. Almost all Republicans, along with some Democrats, oppose the steps Obama has taken to curb greenhouse gas emissions, arguing they will hurt the economy, shutter coal plants and eliminate jobs in power-producing states.

Half the states are suing the administration to try to block Obama's unprecedented regulations to cut power plant emissions by roughly one-third by 2030. The states say Obama has exceeded his authority and is misusing the decades-old Clean Air Act. If their lawsuit succeeds, Obama would be hard-pressed to deliver the 26 percent to 28 percent cut in overall U.S. emissions by 2030 that he has promised as America's contribution.

Opponents also are trying to gut the power plant rules through a rarely used legislative maneuver that already has passed the Senate. A House vote is expected while international negotiators are in Paris.

And Republicans running for president are unanimous in their opposition to Obama's power plant rules; many say that if elected, they immediately would rip up the rules.

The administration mostly has acted through executive power: proposing the carbon dioxide limits on power plants, which mostly affect coal-fired plants; putting limits on methane emissions; and ratcheting up fuel efficiency standards for cars and trucks, which also cuts down on carbon pollution.